Incredible Resilience (3.5e Feat)

From D&D Wiki

Benefit: Whenever you fail a saving throw against a death effect or coup de grace attempt, you do not automatically die. Instead, you merely lose an amount of hit points equal to 10% of your normal maximum and you are stunned for 1 round (although this might still kill you). This HP loss is in addition to any damage dealt by the actual effect. Also, whenever you make a Fortitude save or Concentration check against an effect that uses damage to determine the DC (such as a coup de grace which is DC10+damage dealt) you cut the damage in half for the purposes of determining the DC (round down).

Normal: When you fail a saving throw against a death effect or coup de grace (DC10+damage dealt), you instantly die.